The preparation of inorganic glasses using low temperature routes (soft-chemistry) has been described as an extremely interesting alternative to conventional glass manufacturing. It is especially well appropriate for preparing hybrid matrices. These hybrid materials contain a mineral part which coexists with an organic part. The organic part helps synthesis of xerogels especially during the drying step. This approach has been known since several decades and intensive researches have been devoted in the field for the past 20 years. Another interest of the low temperature routes is that one can easily incorporate in the glassy matrix, thermally sensitive doping agent, such as functional molecular systems or nanomaterials which could confer multifunctionalities to the obtained hybrid material. The interactions between the hybrid matrix and the doping agent are controlled by the nature of the organic parts and their concentrations. The properties of the doped xerogel can be influenced by the strength of these interactions.
Silica based materials are suitable host matrices since they combine good thermal and mechanical properties. In addition, they exhibit interesting optical properties which can be used in many applications. So a large variety of materials for optical and optoelectronic applications were developed by trapping active species into the polymeric network of the silica based gels. Two methods for preparing guest-host systems using the sol-gel technique were developed. The first one consists in simply dispersing the active species in the matrix without strong interaction between the guest system and the silica backbone. In the second approach, the guest active units are strongly bonded to the silica network (Adv. Mater., 2003, 15(23), 1969). The main drawback of the first method is that the solubility of the guest system (organic or organometallic molecules, inorganic nanomaterials) is often low in the more or less polar SiO2-based gels and xerogels. Hence, chemical covalent grafting of the molecular species to the silica backbone can be suitable for greatly increasing their concentrations in the range of 0.1-0.5 M or higher.
The bonding of the doping agent can be accomplished via tri-alkoxysilyl groups in the organic molecular framework, which are hydrolysed and subsequently co-condensed with the silicon alkoxide during the sol-gel process. In Adv. Funct. Mater. 2009, 19, 235-241, the inventors of the present invention described the preparation by sol-gel technique of glass materials from di(arylethylnyl)diphosphinePt(II) complexes functionalized with siloxane groups on the peripheral aromatic rings. Concomitantly the modification of the matrix using alkyl substituted alkoxides conferring hydrophobicity to the network can improve the compatibility between the two systems. Nevertheless, the grafting may induce structural changes on the functional doping agents. Moreover the chemistry of the doping agent remains complicated in most cases.
Another general technical problem, in both approaches, concerns the control of the drying step which has to be very slow to prevent the matrix from cracking. So, different solutions have been developed. One of them has proposed to use additives, such as formaldehyde, in order to control the ultrastructure of the gel solid and pore phases. Gelation, aging, drying, and densification of the sol-gel derived monoliths may be performed rapidly in tens of hours instead of tens of days without cracking (U.S. Pat. No. 4,851,150). However, this implies the contamination of the final materials as the result of the presence of by-products.